Nanomaterials, i.e. structures with at least one dimension between 1 nm and 100 nm, includes a host of substances, are fundamentally interesting due to their fascinating size-dependent optical, electronic, magnetic, thermal, mechanical, chemical, and physical properties, which are distinctive not only from their bulk counterparts but also from the atomic or molecular precursors from whence they were derived (Xia et al., Adv. Mater. 2003, 15, (5), 353-389; Mao et al., Small 2007, 3, (7), 1122-1139). In particular, semiconducting metal sulfide nanoparticulates possess novel optical and electrical properties and are considered as building blocks for photovoltaic devices including dye-sensitized cells, all-inorganic nanoparticle solar cells, and hybrid nanocrystal-polymer composite solar cells in addition to lasers and waveguides.
In recent years, high-quality semiconducting one-dimensional (1D) nanostructures, such as nanowires and nanotubes, with their inherent anisotropy, have been considered as model systems for the efficient transport of electrons and optical excitations. As such, semiconducting nanowires have been used as building blocks for a number of nanoscale energy-conversion, photonic, and electro-optical devices (including field-effect transistors, light-emitting diodes, logic gates, lasers, waveguides and solar cells), as well as electronic circuits (Sun et al., Chem. Int. Ed. 2008, 47, 3215-3218; Wang et al., Science 2001, 293, 1455). A significant amount of effort has been expended in attempting to overcome numerous challenges associated with the goal of achieving a controlled synthesis of semiconducting nanowires with reproducible morphology, crystallinity, chemical composition, and monodispersity.
Prior literature has suggested that the fabrication of PbS, CuS, and CdS nanorods can occur either by electrodeposition or injection of reactants within the channel pores of either anodic aluminum oxide or mesoporous silica templates. (Chen et al., Surf Sci. 2007, 601, 5142-5147; Singh et al., Chem. Mater. 2007, 19, 2446-2454; Routkevitch et al., Chem. Phys. 1996, 210, 343-352; Suh et al., Chem. Phys. Lett. 1997, 281, 384-388; Li et al., Chem. Mater. 1999, 11, 3433-3435; Xu et al., Pure Appl. Chem. 2000, 72, 127-135; Xu et al, Adv. Mater. 2000, 12, 520-522; Thiruvengadathan et al., O. Chem. Mater. 2005, 17, 3281-3287; Gao et al., Adv. Mater. 2003, 15, 739-742; Gao et al., Nano Lett. 2001, 1, 743-748.) As drawbacks in terms of sample quality and reaction conditions, though, nanostructures synthesized using this traditional template method are often either polycrystalline or necessitate an additional annealing step at high temperature.
It would be desirable to develop a protocol that allows for a green, cost-effective methodology of metal sulfide 1-D nanoscale synthesis without the need to sacrifice on sample quality, crystallinity, monodispersity, and purity. That is, it would be a great advance to develop a protocol aimed at metal sulfide nanowire/array formation which would overcome (i) the high-temperatures, (ii) the need for expensive equipment, (iii) the use of potentially toxic, gaseous precursors and byproducts, (iv) the utilization of costly catalysts and performance-altering capping agents (including surfactants), and/or (v) the polycrystallinity of the ultimate product, characteristic of prior art methods.
Additionally, manipulable nanoscale luminescent materials, many of which are either fluorescent, magnetic, or both, are increasingly being used for a number of significant biological applications including drug and gene delivery, biosensing, and bioimaging (De et al., Adv. Mater. 2008, 20, 4225-4241). However, the application of rare-earth phosphate nanostructures as biological labels for in vivo bioimaging purposes has not as yet been demonstrated.
Also, many drawbacks are associated with the synthesis of rare-earth phosphate nanostructures. For example, lanthanide phosphate (LnPO4) nanorods, measuring 20-70 nm in length with aspect ratios from 2 to 7, have been synthesized by calcination of a sol-gel at 400° C. (Rajesh et al., Microporous Mesoporous Mater. 2008, 116, 693-697). Electrospinning has been used in conjunction with the sol-gel process as well to yield polycrystalline nanowires ranging from 60 to 300 nm, after calcination at 650 to 750° C. (Hou et al., Chem. Mater. 2008, 20, 6686-6696; Xu et al., J. Phys. Chem. C 2009, 113, 9609-9615). Generally, the hydrothermal methodology has been primarily used for the synthesis of 1D LnPO4 nanostructures, measuring typically 20-60 nm in diameter with lengths from several hundred nm to several microns. The treatment usually involves reaction in a Teflon-lined stainless-steel autoclave often under anomalous pH conditions, at a relatively high temperature (in the range of 150-240° C.), and involving a reaction times ranging from a few hours up to several days, depending on the experimental circumstances (Chen et al., J. Phys. Chem. C 2008, 112, 20217-20221; Fang et al., Cryst. Growth Des. 2005, 5, 1221-1225; Cao et al., Nanotechnology 2005, 16, 282-286, Yu et al., Mater. Lett. 2007, 61, 4374-4376; Lam et al.; J. Cryst. Growth 2007, 306, 129-134; Yan et al., Chem.—Eur. J. 2005, 11, 2183-2195; Chen et al., J Phys. Chem. C 2008, 112, 16818-16823; Zheng et al., J. Cryst. Growth 2005, 280, 569-574; Fang et al., J. Am. Chem. Soc. 2003, 125, 16025-16034; Yan et al., Solid State Commun. 2004, 130, 125-129; Yu et al., J. Phys. Chem. B 2004, 108, 16697-16702). The synthesis of well-defined crystalline CePO4 nanowires with a diameter of 3.7 nm was reported by use of a microemulsion reaction medium, but could take as long as a month to produce, while ultrasound irradiation of an inorganic salt aqueous solution has been reported for the synthesis of CePO4: Tb and CePO4: Tb/LaPO4 core/shell nanorods (Xing et al., J. Phys. Chem. B 2006, 110, 1111-1113; Zhu et al., Nanotechnology 2006, 17, 4217-4222).
Hence, there is a need for a more facile, milder, less technically demanding, but more cost-effective approach towards the generation of sulfide and phosphate nanostructures.